Question

我有一个包含单个基本类型的泛型类型，以赋予它值相等语义

public class ValueObject<T>
{
    public T Value { get; }
    public ValueObject(T value) => Value = value;

    // various other equality members etc...
}

使用方式如下：

public class CustomerId : ValueObject<Guid>
{
    public CustomerId(Guid value) : base(value) { }
}

public class EmailAddress : ValueObject<string>
{
    public EmailAddress(string value) : base(value) { }
}

问题在于序列化类型：

public class Customer
{
    public CustomerId Id { get; }
    public EmailAddress Email { get; }

    public Customer(CustomerId id, EmailAddress email) 
    { 
        Id = id;
        Email = email;
    }
}

从ValueObject<T>继承的每个对象都包含在Value属性中（如预期的那样）。例如

var customerId = new CustomerId(Guid.NewGuid());
var emailAddress = new EmailAddress("some@email.com");

var customer = new Customer(customerId, emailAddress);

var customerAsJson = JsonConvert.SerializeObject(customer, Formatting.Indented, new JsonSerializerSettings
{
    ContractResolver = new CamelCasePropertyNamesContractResolver() 
})

结果

{
  "id": {
    "value": "f5ce21a5-a0d1-4888-8d22-6f484794ac7c"
  },
  "email": {
    "value": "some@email.com"
  }
}

有没有办法编写自定义JsonConverter，因此Value属性被排除在子类ValueObject<T>的类型中，以便上面的示例输出

{
  "id": "f5ce21a5-a0d1-4888-8d22-6f484794ac7c",
  "email": "some@email.com"
}

我希望有一个JsonConverter可以处理所有ValueObject<T>，而不必为每个JsonConverter子类定义单独的ValueObject<T>

我的第一次尝试是

public class ValueObjectOfTConverter : JsonConverter
{
    private static readonly Type ValueObjectGenericType = typeof(ValueObject<>);
    private static readonly string ValuePropertyName = nameof(ValueObject<object>.Value);

    public override bool CanConvert(Type objectType) =>
        IsSubclassOfGenericType(objectType, ValueObjectGenericType);

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        // converts "f5ce21a5-a0d1-4888-8d22-6f484794ac7c" => "value": "f5ce21a5-a0d1-4888-8d22-6f484794ac7c"
        var existingJsonWrappedInValueProperty = new JObject(new JProperty(ValuePropertyName, JToken.Load(reader)));
        return existingJsonWrappedInValueProperty.ToObject(objectType, serializer);
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        // to implement
    }

    private static bool IsSubclassOfGenericType(Type typeToCheck, Type openGenericType)
    {
        while (typeToCheck != null && typeToCheck != typeof(object))
        {
            var cur = typeToCheck.IsGenericType ? typeToCheck.GetGenericTypeDefinition() : typeToCheck;
            if (openGenericType == cur) return true;

            typeToCheck = typeToCheck.BaseType;
        }

        return false;
    }
}

Answer 1

您可以使用类似于 custom JsonConverter 中显示的Json.Net: Serialize/Deserialize property as a value, not as an object来执行此操作。但是，由于ValueObject<T>没有非泛型方法来获取和设置Value作为对象，因此您需要使用反射。

这是一种方法：

class ValueConverter : JsonConverter
{
    static Type GetValueType(Type objectType)
    {
        return objectType
            .BaseTypesAndSelf()
            .Where(t => t.IsGenericType && t.GetGenericTypeDefinition() == typeof(ValueObject<>))
            .Select(t => t.GetGenericArguments()[0])
            .FirstOrDefault();
    }

    public override bool CanConvert(Type objectType)
    {
        return GetValueType(objectType) != null;
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        // You need to decide whether a null JSON token results in a null ValueObject<T> or 
        // an allocated ValueObject<T> with a null Value.
        if (reader.SkipComments().TokenType == JsonToken.Null)
            return null;
        var valueType = GetValueType(objectType);
        var value = serializer.Deserialize(reader, valueType);

        // Here we assume that every subclass of ValueObject<T> has a constructor with a single argument, of type T.
        return Activator.CreateInstance(objectType, value);
    }

    const string ValuePropertyName = nameof(ValueObject<object>.Value);

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        var contract = (JsonObjectContract)serializer.ContractResolver.ResolveContract(value.GetType());
        var valueProperty = contract.Properties.Where(p => p.UnderlyingName == ValuePropertyName).Single();
        // You can simplify this to .Single() if ValueObject<T> has no other properties:
        // var valueProperty = contract.Properties.Single();
        serializer.Serialize(writer, valueProperty.ValueProvider.GetValue(value));
    }
}

public static partial class JsonExtensions
{
    public static JsonReader SkipComments(this JsonReader reader)
    {
        while (reader.TokenType == JsonToken.Comment && reader.Read())
            ;
        return reader;
    }
}

public static class TypeExtensions
{
    public static IEnumerable<Type> BaseTypesAndSelf(this Type type)
    {
        while (type != null)
        {
            yield return type;
            type = type.BaseType;
        }
    }
}

然后您可以将转换器直接应用于ValueType<T>，如下所示：

[JsonConverter(typeof(ValueConverter))]
public class ValueObject<T>
{
    // Remainder unchanged
}

或者在设置中应用它：

var settings = new JsonSerializerSettings
{
    Converters = { new ValueConverter() },
    ContractResolver = new CamelCasePropertyNamesContractResolver() 
};
var customerAsJson = JsonConvert.SerializeObject(customer, Formatting.Indented, settings);

工作样本.Net小提琴＃1 here。

或者，您可以考虑添加非通用方法来以object的形式访问该值，例如像这样：

public interface IHasValue
{
    object GetValue(); // A method rather than a property to ensure the non-generic value is never serialized directly.
}

public class ValueObject<T> : IHasValue
{
    public T Value { get; }
    public ValueObject(T value) => Value = value;

    // various other equality members etc...

    #region IHasValue Members

    object IHasValue.GetValue() => Value;

    #endregion
}

通过这一添加，WriteJson()变得更加简单：

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        serializer.Serialize(writer, ((IHasValue)value).GetValue());
    }

工作样本.Net小提琴＃2 here。

注意：

ReadJson()假设Value<T>的每个子类都有一个公共构造函数，它使用T类型的单个参数。
使用[JsonConverter(typeof(ValueConverter))]将转换器直接应用于ValueType<T>会有更好的性能，因为CanConvert永远不会被调用。有关详细信息，请参阅 Performance Tips: JsonConverters 。
您需要决定如何处理null JSON令牌。它应该导致空ValueType<T>，还是带有空ValueType<T>的已分配Value？
在ValueType<T>的第二个版本中，我明确实施了IHasValue.GetValue()，以阻止在静态类型代码中使用ValueType<T>实例的情况下使用它。
如果您真的只想将转换器应用于类型子类化 ValueObject<T> 而不是ValueObject<T>本身，请{{1添加对GetValueType(Type objectType)的调用：
```
.Skip(1)
```
然后将转换器应用于static Type GetValueType(Type objectType) { return objectType .BaseTypesAndSelf() .Skip(1) // Do not apply the converter to ValueObject<T> when not subclassed .Where(t => t.IsGenericType && t.GetGenericTypeDefinition() == typeof(ValueObject<>)) .Select(t => t.GetGenericArguments()[0]) .FirstOrDefault(); }，而不是直接应用于JsonSerializerSettings.Converters。

Json.NET - 序列化没有属性名称的泛型类型包装器

1 个答案: